This invention relates generally to increasing the reliability of wireless communication systems and more specifically to a system for varying the size of message packets according to the speed of motion of a receiver.
Radio paging systems and other types of wireless message broadcast systems transmit messages to remote receiving devices. For example, U.S. Pat. No. 4,713,808 to Gaskill et al. (Gaskill) describes a time division multiplexed (TDM) data protocol where pager messages are queued into 13.6 millisecond(ms)time slots which are then multiplexed together to form data frames. Each packet transmitted within a time slot contains 260 bits of information.
It is desirable that remote receivers reliably receive the transmitted information in each packet. However, due to a variety of factors, including environmental conditions, the messages contained in some packets are not always successfully received.
FIG. 1 is a graph showing the condition of an FM signal 12 at the receiver location. Signal 12 has spatial variations in signal strength (i.e., burst errors or nulls 14) that occur for discrete periods of time. Nulls 14 represent portions of signal 12 having a substantial loss of signal strength. Information in signal 12 coinciding with nulls 14 will not be successfully received by the target receiver.
A string of message packets 16, as described above in Gaskill, are shown extending along a horizontal axis representing time. Individual packet 18 of packet string 16 reaches the receiver during null 14. The null 14 destroys some or all of the bits in packet 18.
To correct for unsuccessfully received bits, the system in Gaskill includes a block error checking and correction code (ECC) scheme. However, the ECC scheme in Gaskill, can only correct for a limited number of corrupted bits in each packet (e.g., 7%).
To increase the probability of successfully receiving messages, the pager system in Gaskill retransmits the same message several times in each frame. However, retransmitting messages burdens a valuable communication resource, namely, the transmission path bandwidth. Each time a message is retransmitted, an additional portion of the transmission bandwidth is used for the same message instead of first transmission of other messages.
Another problem with simply retransmitting messages is that the burst error that corrupted the first message may also corrupt subsequent transmissions of the same message.
Several techniques have been devised for reducing the effects of nulls in transmission signals. For example, the same message can be transmitted over multiple frequencies. Since drop-out characteristics change according to carrier frequency, it is likely that portions of corrupted messages transmitted at a first frequency could be successfully received at an alternate carrier frequency.
Multiple transmitter stations are located at different physical locations so that the physical origin and signal strength of the message sent from each transmitter is different. The drop-out characteristics for the signals sent from each transmitter station are likely to be different. Thus, it is likely that the message will be successfully received from at least one of the multiple transmitter stations.
Transmitting and receiving the same message at different frequencies, or transmitting the same message from multiple transmitter stations, requires complex transmitter and receiver circuitry making the communication system more expensive to manufacture and operate.
Another technique for reducing the effects of burst errors involves interleaving multiple message packets together thus creating better burst error correction capabilities. Because receivers are portable, the signal drop-out characteristics at the receiver often change. As will be described below, transmitting a single interleaved packet size for varying signal drop-out conditions is not completely effective in minimizing burst error effects.
Accordingly, a need remains for increasing the probability of successfully receiving messages to receivers without using addition signal bandwidth.